A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In the lithographic apparatus as here above specified, the optical elements of the projection system are mounted in a lens housing that is kept at a predefined conditioned environment, usually provided by a gas flow of inert gases. Typically, He gas is used or N2 gas or the like for keeping the lens environment clean and stable which is vital for imaging quality and life span of the projection system. However, such conditioned environment should be separate from the ambient environment, for example, the clean room environment, which, in a sense of the projection system is not clean at all. This causes problems, for example, when a door opens, a pressure drop of some 15 Pa may occur, which results in a momentary pressure difference between the ambient environment en the inner housing of the projection system. Such pressure differences may cause unwanted optical effects, for example, by mechanical deformation of optical elements etc. Although these problems are recognized in the art, and pressure stabilization systems have been developed that are designed to level pressure differences between the gas conditioned environment and the ambient environment, usually, these systems have a large response time which is not adequate for responding to sudden fluctuations. Therefore, generally, the gas conditioned environment follows general pressure fluctuations on a larger time scale, but fails to adequately respond on shorter time scales.